“These are very rare events, and it’s even more rare that we get to watch the whole thing,” he said.

It’s a strange, mysterious world on Mercury.

It’s the innermost planet in our solar system — that is, the closest to the sun — and has no atmosphere. Its temperature can swing from 800 degrees Fahrenheit to minus 200 at night, Professor Kendall said.

Enormous volcanoes erupted across its surface billions of years ago, yet the icy craters at the north pole of Mercury, where the sun never shines, are some of the coldest places in the solar system, he added.

During today’s transit, Mercury will look like a moving freckle on the sun.

But don’t stare straight at it with sunglasses or binoculars: You’ll need the help of a solar telescope, which Professor Kendall and other astronomers have set up across the city.

An earlier version of this article, using information from a William Paterson University professor, misstated the next occurrence of the Mercury transit and described incorrectly a blue moon that is to occur on May 21. The Mercury transit will be in three years, in 2019, not in the next decade or two. And the blue moon will be a seasonal one, not the second blue moon in a calendar month.